An Al-containing steel sheet comprised of iron plus aluminum is an alloy having a high temperature oxidation resistance and a high electrical resistance. Raising the Al concentration can be expected to improve these properties, but simultaneously the toughness falls and the workability remarkably drops. For this reason, the maximum Al concentration at which industrial mass production is possible in the ordinary steel-making and rolling process is just 6.5 mass %. Mass production technology for improving the workability and producing high Al-content steel sheet containing more Al is eagerly awaited.
As prior art for improving the workability of a high Al-content steel sheet, Japanese Patent Publication (B) No. 6-8486 describes adding prescribed amounts of P and REMs to improve the hot workability since the Ce in REMs is present in steel as a fine grain phosphate. However, this requires expensive additive elements and leads to higher costs. Further, the upper limit of the Al concentration described in this publication is 6.5 mass %.
Japanese Patent Publication (A) No. 1-4458 describes making 70% or more of the solidified structure of rapidly quenched ribbon columnar crystals so as to improve the toughness of stainless steel containing Al in a concentration of 1 to 7 mass %. However, considering the fact that special equipment is necessary for producing rapidly quenched ribbon and for the mass producibility of ribbon, the cost again ends up becoming higher.
Therefore, as a method of production of a high Al-content steel sheet, the method of depositing an Al or Al alloy on the surface of a steel sheet having a concentration of Al of less than 6.5 mass % able to be mass produced by the usual steel-making and rolling process and treating this by diffusion heat treatment to increase the Al concentration is known.
For example, Japanese Patent Publication (B) No. 4-80746 discloses, as art for efficiently rolling an Al plated material, a method of production of a metal substrate comprising plating the surface of a steel sheet with Ni, then plating it by Al or an Al alloy, rolling it to the thickness of the final product, then working it and assembling it into a metal substrate and treating it by diffusion heat treatment. Further, Japanese Patent Publication (A1) No. 2003-520906 describes, as a method of production of dimensionally stable Fe—Cr—Al foil steel sheet, coating it with Al or an Al alloy during which making the amount of coating 0.5 to 5 mass % of the entire mass so that the shrinkage in the length and/or width direction becomes 0.5% or less even in the uniform diffusion heat treatment after cold rolling. However, these methods of production do not aim at the effect of improvement of the toughness or workability of the high Al-content steel sheet after diffusion heat treatment.
Japanese Patent No. 3200160 describes, as a method of production of an Fe—Cr—Al alloy foil, depositing Al or an Al alloy on a stainless steel sheet containing a prescribed amount of B, treating it by diffusion heat treatment to make the deposited ingredient diffuse in the steel sheet and obtain a steel sheet containing Al in a concentration of 1 to 10 mass %, then cold rolling it by a reduction rate of 30% or more and annealing it in a range of 800° C. to 1200° C. one time or more so as to promote the uniform diffusion of B and improve the resistance to high temperature embrittlement. With this method of production, the cold rolling and annealing are repeated after the diffusion heat treatment, so the cost becomes higher.